Wireless telephones have received wide acceptance for use in cellular systems, as well as in wireless user premises equipment applications. There are new cellular telephone systems under development, as well as wireless personal communication systems (PCS) for both the licensed and unlicensed bands. A cellular telephone or cell-like communication system involves a network of fixed base stations that provide an integrated communication service to a plurality of mobile transmitter/receiver (“transceiver”) units, e.g., cellular telephones. The communications network attempts to communicate with each transceiver from the base station that provides the optimal communication, such as in terms of signal level, clarity, etc. The optimal base station is usually, but not necessarily, the one nearest the mobile transceiver. To provide the optimal communications support, the network needs to locate the geographic position of the mobile only to the “rough” level required to assign the proper base station.
This rough estimate of the location of a mobile has been a hindrance in extending the conventional 911 aspect of communication service to the mobile environment. When a user makes an emergency 911 call on a standard corded telephone, the location of the user is quickly determined since the physical location of the telephone is known and unchanging. In contrast, cell phone callers can “roam” anywhere within the physical bounds of the entire system and, as result, “permanent” geographic location information associated with the cell phone is meaningless.
Based on this realization, the FCC has defined an “Enhanced 911” requirement which mandates that all wireless service operators must be able to provide geographic position data to Public Service Answering Positions (PSAPs) for E911 calls by October 2001. As a result of this mandate, wireless network operators connecting to the public switched telephone network must implement E911 service in two phases: Phase I stipulates that the system must pass the caller's phone number, cell-site, and cell-sector location information through to a PSAP. Carriers were to complete this step by April 1998, but many are still in the implementation stage. Phase II presents the more challenging task (at least from a location technology standpoint), of providing the 911 caller's location to the appropriate PSAP with an accuracy of 125 meters RMS (root-mean-square), in at least 67% of all cases. As most wireless operators proceed to fulfill Phase I requirements through their networks, they are also assessing which location technologies most effectively meet the Phase II requirements. These positioning methods are generally divided into two categories: (1) network-based systems that require some equipment installation at network base stations; and (2) handset-based systems that add GPS or another location technology to the wireless phone, but generally do not require additional network equipment.
Most network-based caller-location systems are based on time-difference-of arrival (TDOA) or angle-of-arrival (AOA) measurements, or a combination of these two techniques. Cell-Loc provides one exemplary technology to determine the geographic position of mobile stations, as disclosed in U.S. Pat. No. 5,890,068, issued to M. T. Fattouche et al. on Mar. 30, 1999. In this case, receive-only antennas (ROAs) are located at base stations and TDOA measurements are made for various channel transmissions from mobile stations. This information is then used to determine the position of the devices, without requiring alterations to either the base stations or the mobile stations. In AOA technology, a set of receive-only phased array antennas are located at each base station and used to compute the angle at which signals transmitted from a mobile station arrive at the base station. See, for example, U.S. Pat. No. 5,786,791, entitled “Method for determining an angle of arrival of a signal transmitted by a remote unit in a communication system”, issued to E. J. Bruckert on Jul. 28, 1998. A disadvantage of both of these techniques is that the geographic position of a mobile station can only be determined when the mobile station is transmitting. An enhancement to these network-based techniques, referred to as RF fingerprinting, measures the distinct RF patterns and multipath characteristics of radio signals arriving at a cell site from a mobile unit, using this information to determine the mobile's location with an additional level of granularity.
The FCC mandate clearly requires that wireless carriers be able to locate any caller requesting emergency assistance through its network. On the surface, this would appear to eliminate handset-based solutions, such as utilizing a Global Positioning System (GPS), from consideration since it would be essentially impossible to add GPS (or other location-sensitive components) to all phones operating on a network by October 2001. However, it is presumed that such a system could be phased in, with newer phones including the necessary technology. At a recent conference on wireless location system implementation, a Nokia representative reported that the company currently favors adding GPS to the handset as the best solution for caller location on CDMA networks. Ericsson has suggested a short-term solution based on TDOA and a long-term solution combining GPS in the phone with differential corrections, using a network server. In addition to manufacturing wireless phones, both Nokia and Ericsson supply wireless network infrastructure equipment. Lucent Technologies and Qualcomm, which also manufacture both wireless handsets and infrastructure equipment, report that they too are investigating long-term caller location solutions that include the addition of GPS to handsets. These wireless infrastructure suppliers generally favor GPS as an element of the long-term solution based on the view that aided-GPS will support a higher level accuracy will be needed to support a wide range of commercial location-based services. However, a number of major carriers continue to express a preference for a network-based solution.
Regardless of the technology ultimately deployed to provide the position location information of a mobile E911 caller, there are further aspects of this technology that may be deployed to provide additional features to such a system.